The present invention relates generally to the field of parts manufacturing. More specifically, the present invention relates to methods and apparatuses for forming structures using robotic devices without the use of molds.
Although the present invention can be used in any application requiring that a part or structure be fabricated from a certain material without molds, it will be presented herein as it applies to composite materials for convenience in introducing the invention. The present invention, as will be evident from the following discussion, can be used for fabricating parts from any material without the use of molds. The materials used can be any material that (1) can adhere to itself, and (2) can be cured (e.g., by applying heat, pressure, curing agents, plasmas (e.g., charged particles, charged ions, electron beams, etc.)), ultraviolet treatments, some other means of curing, etc.) to maintain its shape.
A composite material, as its name implies, includes two or more distinct materials. The composite exhibits the best properties of the constituent materials. Well-known examples of composite materials include bricks made of mud and straw, and structures made of concrete and reinforcing bars. Composites of ceramic and metal have recently become available.
The composite materials of interest in the field of the present invention include a high-modulus fiber and a polymer binder. The non-fibrous material is called the matrix; examples of the matrix are thermosets, such as epoxy resins, and thermoplastics, such as nylon. The types of matrix are subdivided into thermosets and thermoplastics. Thermosetting materials are chemically and irreversibly altered during the cure process, which often involves the application of heat. In some cases, the heat is the exotherm of the curing reaction. Thermoplastics simply melt when heated; they can be remelted a number of times, though they degrade after a number of heating/cooling cycles.
These materials are available in a number of forms. The fiber can be separate from the matrix material, or it can be already impregnated with it. The latter form is called prepreg. Thermoplastics, which are solid at room temperature, can be commingled, or woven, with the fibers, or preconsolidated, where they are already melted together with the fiber. Preconsolidated and prepreg materials are often sold as rolls of flat tape, which can be dispensed through a dispensing means.
Available methods of forming structures of continuous-fiber polymer-matrix composites (hereinafter "CFPM composites"), which are fiber reinforced, all require mandrels, molds, forms, or dies. Such forms place constraints on the producible shapes. For example, filament winding on a mandrel cannot produce concave objects, and the requirement that the mandrel be removed from the interior of the finished structure makes very difficult the fabrication of shapes having a wider cross-section in the middle than at the ends. The need for mandrels, molds, forms, or dies adds expense to the cost of fabricating a new shape, particularly if only a few copies of the shape are desired.
In contrast to this state of affairs, other materials can be fabricated into complex shapes by various methods of rapid prototyping. These technologies are characterized by the ability to produce shapes of very high complexity directly from an electronic model of the shape, without requiring fabrication of a new form. An example is stereolithography, in which the shape is built up layer by layer, using a computer-controlled laser to selectively cure epoxy resin. To produce a new shape, only the software controlling the curing is changed; no changes to the hardware are necessary. Stereolithography, however, can build only structures composed of pure resin, or resin containing chopped, non-continuous fiber. Another example of rapid prototyping is laser sintering, in which the laser energy bonds powdered metal and/or ceramic. Given the success of these technologies, it is desirable to have a method of rapid prototyping for CFPM composites, so that the domain of rapid prototyping is extended to these high-modulus, low-weight materials.
Such a method would be able to produce shapes of high complexity, not subject to the shape constraints imposed by the need for molds; furthermore, the only change to the system needed to produce a new shape would be to the control software, which would be automatically generated from an electronic model of the shape. Such a method would have the dual advantages of being able to produce shapes currently producible only with great difficulty, and at lower cost because mandrels need not be made.
Fabrication methods for composite structures fall into six basic categories: lay-up, resin transfer molding, filament winding, fiber placement, pultrusion, and braiding/weaving, which are described below. More complete descriptions are available elsewhere, e.g., C. A. Harper, Handbook of Plastics, Elastomers, and Composites, 2.sup.nd Ed., McGraw-Hill, New York, Chapter 5 (1992).
Lay up
In the lay-up process, fiber and matrix material are placed in a mold. The fiber and matrix can be applied separately or simultaneously. Once the placement is complete, the resin is cured. This can be at room temperature and pressure, but better results are obtained when both quantities are elevated in an autoclave or press.
In hand lay-up, fiber mats are placed, resin is sprayed or painted on, and is pressed into the fiber with rollers or squeegees. Then, the material cures at room temperature.
Fiber and resin can be combined before lay-up in several ways. In spray lay-up, fiber is cut and combined with resin in a spray gun, which is then used to coat the mold. Continuous fiber-reinforced "prepreg" (sheets of uncured resin impregnated onto continuous reinforcement fibers) materials can also be used, which materials are used in the manufacturing of composite structures in the aerospace and automotive industry. When prepeg materials are used, automated lay-up becomes easier, in the form of automatic tape lay-up. The system described by Olsen and Craig includes a robot mounted prepreg tape dispenser (see H. B. Olsen and J. J. Craig, Automated Composite Tape Lay-Up Using Robotic Devices, Proceedings, 1993 IEEE International Conference on Robotics and Automation, IEEE Computer Society Press, Los Alamitos, Calif., Vol. 3, pp. 291-297 (1993)). The dispensing head is capable of cutting the tape, restarting the dispensing process, and applying pressure at the point of application. It differs from filament winding in this ability to stop dispensing tape (by cutting) and restart at a different point.
Resin Transfer Molding
The hallmark of resin-transfer molding ("RTM") is the injection of matrix resin into a closed mold which contains the fiber. Curing takes place in the mold. The fiber can be woven or braided into an approximation of the final shape, or preform, before being placed into the mold.
Filament Winding
Filament winding can produce very large shapes, provided that the curvature is everywhere convex. A filament winding machine consists of a rotating mandrel and a fiber dispensing head that travels the length of the mandrel. Synchronized with the mandrel rotation, the head can change the angle of the fiber with respect to the mandrel axis, so that helical plies optimized to handle expected loads can be laid down. Filament winding machines are programmable, so that different winding patterns can be specified. The mandrel can also be changed, allowing even more flexibility.
The three main constraints imposed by the filament winding process, in order of increasing difficulty to overcome, are: (1) the mandrel must be removed from the interior of the complete structure; (2) the object must have positive curvature everywhere; and (3) the mandrel must be fabricated. If the shape does not allow the mandrel to be removed intact (e.g., a tube with wider diameter in the middle than at the ends), it can be removed destructively. Plaster and salt mandrels have been used for this purpose.
An object with re-entrant curvature can be filament wound if the winding is followed by suitable post-processing, such as hand lay-up. A disadvantage, however, is that the requirement for a mandrel cannot be avoided.
Fiber Placement
The fiber placement technique was described above. It is an improvement on filament winding in that it can produce shapes with reentrant curvature. However, it still requires a mandrel, and therefore retains the associated disadvantages. Furthermore, the cost of fiber placement machines is very high.
Pultrusion
Pultrusion is the most economical fabrication method for objects having constant cross-section. The fiber and uncured resin are pulled through a heated die which simultaneously shapes and cures the product. Beams and driveshafts are examples of products well-suited to this process.
Note that while this process does not require a mold, a die of the cross-sectional shape is needed.
Braiding and Weaving
The braiding and weaving technique was mentioned in the discussion above of RTM. Dry fibers are braided or woven into configurations optimized for the expected load, and approximately the shape of the target. The resulting network is placed in a mold, impregnated with resin, and cured. The braiding and weaving is performed by programmable machines.
A wide range of shapes can be produced with the same equipment, and smooth transitions from one shape to another are possible. A mold is still required for resin impregnation and curing.
The present invention is a method of and apparatus for producing structures composed of CFPM composites that do not require a mandrel, and are therefore not subject to constraints on producible shapes. This allows shapes to be produced that are otherwise difficult to make using conventional methods. For example, a cylinder-like object, with a square cross-section in the middle and circular cross-section at the ends, can be made with the method of the present invention, but filament winding would require a destructively-removable mandrel. Furthermore, the method of the present invention is implemented with programmable devices, allowing automatic programming of the system to produce a shape from an electronic model of it. The method is the basis of a CFPM rapid prototyping system as envisioned in the discussion above.